This invention relates in general to an apparatus and method for the clarification of water where the removal of suspended particles is accomplished by a flocculating agent and flotation using a stream of rising, microscopic air bubbles. More specifically, it relates to apparatus and a process for dissolved air flotation clarification using multiple sequential zones of clarification, clarified water removal through an air pocket, and improved removal of floated sludge.
Water clarification, the removal of particulate contaminants suspended in water, is used to treat waste water from manufacturing processes, particularly in the paper and pulp industries, in the treatment of municipal water drinking supplies, and in sewage treatment. The water can be clarified by sedimentation or flotation of the particles. Known sedimentation techniques and apparatus are efficient, but are limited even at peak efficiency to a comparatively slow sedimentation rate, about 0.5 gallons per minute per square foot (20 liters/min/m.sup.2). To handle large volumes of raw input water, sedimentation facilities must therefore be large, with attendant cost and space utilization disadvantages. Also, sedimentation tanks are typically much deeper than tanks for flotation clarification and the sludge removed has a lower solids content than with flotation clarification.
Flotation techniques dissolve a few percent of air by volume in pressurized water and then release the air in the form of microscopic bubbles which attach to the particles and carry them upwardly to the surface where they form a floating sludge. This general type of clarification is sometimes termed dissolved air flotation or "DAF". The particles are usually coagulated and flocculated using conventional agents such as alum and/or polymers before the air bubbles are introduced. Flotation techniques are theoretically capable of achieving clarification rates of 7.5 gallons per minute per square foot (GPM/SQFT) of flotation area (300 l/m.sup.2 /min). Heretofore in practice the rates have been less than this theoretical value, but significantly better than for sedimentation techniques.
Several early attempts by applicant to use flotation techniques to clarify water are described in U.S. Pat. No. 2,874,842 issued in 1959 and U.S. Pat. No. 3,182,799 issued in 1965. They used a stationary tank with no skimmers or other moving components in the tank. The gas bubbles were introduced via the main raw water inlet and guided within the tank by an internal deflector ('842) or a stack of internal baffles ('799). Because the gas bubbles were guided by stationary components, there was no design problem created by the turbulence of moving part in the flotation tank. Also, these devices did not lend themselves to treatment at high flow rates. The '799 apparatus had the additional problems in that (i) the inlet water had to be separately fed from the side to the region between each adjacent pair of baffles and (ii) the flow paths for floated particles varies depending on the vertical position of the associated baffles defining the flow path. This latter situation means that the apparatus either does not fully treat the inlet water, or is slow.
Applicant holds several other U.S. patents for water clarification apparatus and processes, including U.S. Pat. Nos. 4,022,696; 4,377,485; 4,626,345; 4,184,967;and 4,931,175, all of which greatly improve over the performance of the early'842 and '799 devices. In the '696 clarifier, which continues to be sold under the trade designations "SPC" and "Supracell", the flotation occurs in a circular tank. The raw water is fed into the tank via a central pipe, a hydraulic joint, and an inlet pipe with multiple outlets immersed in the tank which rotates about the tank. The inlet flow is oriented opposite to the direction of rotation of the inlet pipe and is at a velocity with respect to the rotation rate such that the raw water has a net zero velocity as it enters the tank. The raw water inlet flow assembly and a scoop for removing the floated sludge are mounted on a heavy carriage driven to rotate about the tank. The scoop is preferably a multi-bladed one of the type described in applicant's U.S. Pat. No. 4,184,967. It has a motor mounted on the carriage to rotate the scoop. The rate of rotation of the carriage about the tank is set so that the floated particles will reach the surface of the water held in the tank in the time of one rotation. A good degree of clarification can be achieved with each rotation of the carriage using a comparatively shallow tank, e.g., 16-18inches. This SPC unit clarifies at a rate of about 130 liters/m.sup.2 /min (3.25 gal/min./sq.ft.). This is much better than the rate using sedimentation techniques, but less than half of the theoretical maximum rate.
In the "Supracell" and the other clarifiers described in the aforementioned patents, there are common design features and limitations. For one, the flotation of the flocked particles is generally vertical (or follows inclined conical plates), and is in a body of water that is comparatively shallow and generally free from turbulence. Second, in all of these designs the removal of the floated sludge is by a bladed, rotating scoop feeding an inclined discharge pipe. Third, in each design clarified water is collected by pipes or hollow headers with inlet apertures. The pipes are stationary or rotating in the water at the bottom of the tank. In units using a second filtration stage, e.g. a bed of sand at the bottom of the clarification tank, the pipes can be embedded in the sand bed. Also, heretofore, in order to increase the capacity of a given type of clarifier, one built a larger diameter tank. While in theory one can build a large enough tank to accommodate any clarification load, cost and space constraints have provided practical limitations on the capacities of these units.
More recent approaches to increasing clarification capacity, while retaining physical compactness and a competitive cost of manufacture and operation, are described in U.S. Pat. Nos. 5,268,099; 5,296,149; 5,320,750; 5,415,771; and 5,306,422, all to the present applicant as sole or joint inventor. The '149 patent describes a clarification sold under the trade designation "SPC-L" as an improvement over the '696"Supracell" brand clarifier, as well as the '175 conical plate or "SPC-P" clarifier. The SPC-L clarifier substantially fills a shallow tank with intersecting radial and conical concentric plate-like lamellae. They form an annular array of inclined, open-ended channels which each lock and transport a column of water to be treated. The lamellae rotate slowly in a tank between a fixed raw water distribution header and a fixed clarified water withdrawal header. Rotation of the lamellae hydraulically couple a floated sludge layer to rotate the sludge onto a ramp-like lip of a fixed, hollow sludge skimmer. A screw conveyor mounted within the skimmer rotates to advance the collected sludge radially across the clarifier tank to a discharge pipe. This clarifier operates with one stage in a shallow tank that is substantially filled with the lamellae.
U.S. Pat. No. 5,306,422 describes a clarifier using a double stack of DAF clarifiers where the output of the upper clarifier is fed to the lower clarifier for further clarification. Clarification occurs sequentially in two separate clarifiers with the output of one clarifier fed by gravity to the underlying clarifier in the stack.
U.S. Pat. No. 5,415,771 describes a high capacity clarifier with a single large diameter tank that is divided into multiple sectors by rotating raw water distribution headers and associated clarified water collection headers. Rotating bladed scoops associated with each of these plural header sets remove the floated sludge. Again, clarification occurs in a single stage in a single shallow tank.
U.S. Pat. No. 5,320,750 described an improvement on the '345 clarifier (sold under the trade designation "SASF") which has a central flocculator with a spill-over feed to a surrounding DAF clarification tank with a filter media, such as sand, located at the bottom of the tank. This'750 clarifier, sold under the trade designation "SAF-BP", isolates the first filtrate and uses it to backwash the sand bed sector-by-sector. Clarified water is collected via a network of apertured pipes embedded in the sand layer. Floated sludge is removed by a rotating bladed scoop. The patent also discloses a valve, sold under the trade designation "KD", that introduces air bubbles to the treated water in the central flocculator.
The '696"Supracell" brand clarifier first introduced in the 1970's continues to be a standard, well-received product for many applications. There remains, however, a need for a compact, non-filtering clarifier which offers performance, maintenance and cost advantages over the "Supracell" clarifier.
It is a principal object of this invention to provide a DAF-type clarifier and process with an enhanced clarification efficiency and capacity as compared to a comparably-sized "Supracell" clarifier.
A further principal object is to provide a sludge removal apparatus and method which avoids the use of rotating bladed scoops and carriages that rotate around the tank to transport the scoop, and, in some instances, also to introduce treated water to a clarification tank.
Another principal object is to provide the foregoing advantages while reducing power consumption as compared to a comparably-sized "Supracell" clarifier.
Another object is to provide a clarifier with the foregoing advantages which can be enclosed.
Still another object is to provide such a clarifier which is also compact.
A further object is to provide a clarifier with the foregoing advantages which has a favorable cost of manufacture.